The present invention relates in general to communication networks, and is particularly directed to a mechanism for automatically determining the channel availability and DS0 time slot transmission assignment order of bearer and data channels that may be employed by ISDN interface circuit cards, such as U-Basic Rate-One Transmission Extension, or U-BRITE, ISDN circuit cards (also known in the industry as Basic Rate Interface Transmission Extension (U-BRITE) cards), for transporting a plurality of digital subscriber line (DSL) channels between geographically separated sites where ISDN channe-linterfacing channel banks are installed.
Integrated services digital network (ISDN) communication systems enable telephone service providers to supply multiple types of signalling channels from a central office to a network termination interface at a customer premises site. An example of a reduced complexity xe2x80x98extended distancexe2x80x99 ISDN communication network architecture is diagrammatically illustrated in FIG. 1 as comprising a PCM communication link (such as a T1 data rate (1.544 Mb/s) optical fiber link) 10, through which a channel bank 20 at a xe2x80x98westxe2x80x99 end of the PCM link 10 transmits and receives signalling traffic with respect to a customer premises equipment (CPE) served by a channel bank 30 at a remote or xe2x80x98eastxe2x80x99 end of the PCM link 10. The channel bank 20 at the west end is coupled by way of a link 25 to a central office switch 21 (such as a 5ESS switch manufactured by ATandT), and includes a line interface unit (LIU) 21 that is coupled to the PCM link 10. The channel bank 30 at the east end has an LIU coupled to the PCM link 10 and is coupled by way of a local loop 35 to a customer premises 40.
As shown in FIG. 2, in order to provide service to remote customers at customer premise sites 40, the channel bank 30 at the east end of the PCM link 10 has a plurality of U-BRITE circuit cards 33 installed in the channel bank unit""s backplane 34. Each respective U-BRITE circuit card 33 is dedicated to providing extended ISDN service to remote customer premises equipment via a local loop 35, between the U-BRITE circuit card and digital communication equipment 40 installed at a respective customer premises.
A carrier system transceiver within a line interface unit (LIU) is operative, under control of an attendant communications control processor to transmit and receive standard 2B+D ISDN data traffic over the PCM digital data link 10. To interface digital subscriber loop (DSL) over the local loop (twisted pair) 35 to the customer premises equipment (CPE) 40, the U-BRITE circuit card 33 also includes a line transceiver and an associated line interface, which are also operative, under internal microprocessor control, to interface PCM data with the line interface unit 31 and to transmit and receive (basic rate 2B1Q) ISDN signals over the local loop 35 to and from CPE 40.
For transporting basic rate (2B+D) ISDN channels, the communications industry standard TR-TSY-000397 multiplexing format of a respective DSL channel conveyed by the T1 link is a DS0 byte triplet, accommodating a pair of bearer channel time slot octets B1 and B2 and a data channel time slot octet D. As diagrammatically illustrated in FIG. 3, within a respective DSL time slot, these three DS0 channel octets are typically ordered such that the bearer octet B1 is transmitted first, followed by the data octet D and then the second bearer octet B2, although the order may change depending upon the equipment vendor.
For example, some vendor""s cards switch the order of the second and third octets, such that the data channel octet D immediately follows the second bearer channel octet B2, which immediately follows the first transmitted bearer channel octet B1, as shown in FIG. 4. Other vendor""s cards switch the order of the first and second octets, such that the data channel octet D immediately precedes follows the first bearer channel octet B1, which immediately precedes the second bearer channel octet B2, as shown in FIG. 5. Regardless of the multiplexing order chosen, it is customary industry practice that the B1 channel is assigned a DS0 time slot ahead of that of the B2 bearer channel. Namely, the B2 channel, if used, will follow either directly after the D channel (as shown in FIG. 3), or after the B1 channel (as shown in FIGS. 4 and 5).
Regardless of the multiplexing order chosen, whether or not a respective DS0 octet/channel is enabled within a particular DSL time slot is customarily defined by manually presetting each of a set of three option switchesxe2x80x94one for each channel (B1, B2 and D)xe2x80x94on the ISDN interface card. As a consequence, a not infrequent problem faced by a telecommunication service provider is the failure of an installer to have properly set the DS0 option switches on one or more ISDN interface (e.g., U-BRITE) cards in accordance with their intended ISDN channel assignments. Namely, unless the DS0 octet time slot assignment switches are properly optioned, then when the ISDN interface cards attempt to communicate with one another by way of a respective DSL time slot, they may encounter different DS0 time slot octet assignments, resulting in a lack of DS0 multiplexing synchronization between the two cards and transmission failure. A conventional solution to this problem, which is both labor intensive and time consuming, has been to dispatch a service technician to the remote site to physically examine and manually change the DS0 time slot option switch settings.
In accordance with the present invention, this conventional xe2x80x98travelling technicianxe2x80x99 approach to solving the above-described DS0 time slot misconfiguration problem is successfully remedied by a software-based DS0 channel multiplexing format analysis routine exercised by the ISDN interface card""s microcontroller, so as to automatically determine what DS0 channels are available at the remote ISDN circuit card and the order in which those DS0 channels are multiplexed by that remote card""s circuitry.
For this purpose, the DS0 time slot monitoring routine of the present invention initially supplies a continuous stream of xe2x80x98onexe2x80x990 bits as data to be transmitted to each of the three available DS0 channels of the DSL time slot of interest, so that an error detection mechanism employed by the ISDN card, such as a cyclic redundancy check (CRC) mechanism, operating on each data stream will cause an xe2x80x98all onesxe2x80x99 associated code to be transmitted in the D octet portion of a respective DSL time slot of interest to a far end card that is potentially installed in the backplane card slot associated with the DSL time slot of interest.
A given assumption is that, if a remote ISDN card is installed for a DSL time slot of interest, at least one bearer channelxe2x80x94the B1 channel if only one bearer channel is used and the D channel will be enabled for that DSL time slot. The multiplexing order of the DS0 time slots may be initially established at a prescribed default order, such as B1-D-B2, as shown in FIG. 3, described above.
For this initial default order, the contents of the returned DSL time slot of interest are monitored to determine whether the second DS0 time slot contains the CRC data expected to be returned over the D channel from the far end card. If so, it is concluded that the second DS0 time slot is also used for D channel signalling by the far end card. The routine then proceeds to determine whether or not the second bearer channel B2 is enabled.
For this purpose, the all one""s data previously asserted for the second bearer channel B2 is replaced with a prescribed (not all one""s) data sequence, which causes the CRC operator in the transmitting card to compute a CRC code different from that for the original all one""s sequence. This CRC code is then transmitted over the D channel to the far end circuit. The receiver at the far end card recalculates its own CRC code on the data. If the two CRC codes match, a far end blocking error code (FEB bit) returned by the far end card is set to a first binary state (e.g.,) indicating no error, and it is inferred that the DS0 channel assignments of the far end card correspond to those of the near end card, and the routine is terminated.
However, if the two CRC codes do not match, the far end blocking error bit is set to a second binary state (e.g., 0) indicating an error. It is then inferred that the third available DS0 channel is not used by the far end card for B2 bearer channel signaling, and the B2 channel is turned off.
If the initial monitoring step reveals that the second DS0 time slot is not used for D channel signalling by the far end card, then the multiplexing order is rearranged to assign the third DS0 channel for D channel signaling, and the contents of the third DS0 time slot are monitored to determine whether it contains the CRC data expected to be returned over the D channel from the far end card. If the third DS0 time slot is used for D channel signalling by the far end card, then it is known that the DS0 channel assignment order is the order B1-B2-D and the routine is terminated.
However, if the third DS0 time slot is not used for D channel signalling by the far end card, then it is inferred that the DS0 channel order employed by the far end card is D-B2 and the multiplexing order is rearranged. For this second alternative DS0 time slot order (D-B2), the contents of the first DS0 time slot of the returned DSL time slot of interest are monitored to determine whether that first DS0 time slot contains the CRC data expected to be returned over the D channel from the far end card.
If a D channel synchronization failure again occurs, it is concluded that there is no ISDN interface card installed at the remote site for a DSL time slot of interest, and the routine is reinitiated for the next DSL time slot. However, if the D channel is detected as the first time slot, then the B1 channel is turned off, and the all one""s data previously asserted for the second bearer channel B2 is replaced with a prescribed (not all one""s) data sequence. The D channel from the far end card is then monitored for the return of a new CRC code which should match that computed on the not all ones data due to the change in the data being placed on the B2 channel. If there are no far end blocking errors, it is inferred that the DS0 channel assignments of the far end card correspond to those currently used by the near end card, and the routine is terminated. However, if far end blocking errors are detected, it is inferred that the third available DS0 channel is not used by the far end card for B2 bearer channel signaling, and the B2 channel is turned off.